Conventionally, a sensor is mounted on a predetermined position and has a maximum sensing angle of 180 degrees. Therefore, it is necessary to mount at least two sensors at individual positions in order to cover a desired sensing angle of 360 degrees. The known sensor arrangement has a problem in that in order to electrically wire each of the sensors, it is necessary to provide a plurality of wires. Further, the known sensor arrangement is time consuming to install, thereby increasing labor costs. In addition, in wiring a building for the sensors, it may be necessary for the wall of the building where the sensors are to be arranged to be partially broken and this causes the partially broken wall to be repaired after the sensor wiring is completed, thereby further increasing the installation cost of the sensors. The sensors may be installed on the wall of the building in such a manner that the wires of the sensors are exposed to the outside of the wall, thereby obviating the need to partially break a wall. However, a problem is attendant with this type of sensor wiring in that the exposed wires may be easily short-circuited or disconnected due to the exposed nature of the wiring. Furthermore, such external wiring provides a poor appearance.